Skip to main content

Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

See other formats


HYDROFLUORIC ACID

27

For the quantitative determination of arsenic in the crude acid, 20 c.c.
are diluted with as much water and approximately neutralised with sodium
carbonate. A little ammonia and then yellow ammonium sulphide are
added and after acidifying with pure hydrochloric acid, the liquid is heated
on a steam-bath and a current of hydrogen sulphide passed through it for
2 hours. The precipitated arsenic sulphide is collected, washed, dissolved
in potassium hydroxide solution, oxidised with bromine, precipitated with
magnesia mixture and weighed as magnesium pyroarsenate. I gram
of the pyroarsenate = 0-48387 gram As.

4.  Metals.—10 c.c., diluted with 50 c.c. of water, are treated with
hydrogen  sulphide,  with ammonia and ammonium sulphide, and with
ammonium oxalate ;   pure acid exhibits no change with these reagents.
Iron is also easily detected by means of potassium feirocyanide (deep blue
coloration or precipitate).

5.  Chlorine.—5 c.c. of dilute, fresh starch paste are treated with a
few drops of 10% potassium iodide solution absolutely free from iodate
and a few drops of dilute sulphuric acid.    On addition of i c.c. of the hydro-
chloric acid, diluted with water, a blue coloration forms in presence of
free chlorine.

6.  Bromine and Iodine or the Corresponding Acids.—20 c.c. of
the acid are neutralised with soda and evaporated to dryness, the residue
being taken up in a little water, a little fresh chlorine water added and the
liquid shaken with carbon disulphide ;   in presence of bromine or iodine
the carbon disulphide becomes yellow or violet.

7.  Determination of the Hydrochloric Acid.—With the pure acid,
it is sufficient to determine tlie acidity with a standard alkali (i c.c. N-alkali
= 0-0365 gram HC1) or to measure the specific gravity and then deduce
the acid content by means of tables.    When the acid is not pure, the esti-
mation, is made by precipitating with silver nitrate either gravimetrically
or volumetrically by Volhard's method (see p. 10).

*
* *

The crude acid may contain marked amounts of sulphuric acid (up to about
9% SO3), but for technical purposes the content should not exceed 1-5%. Con-
siderable quantities of arsenic (up to 10 grams per 100 kilos, according to Buch-
ner) may also be found and small amounts of iron. The pure acid may contain
traces of sulphuric acid which should not, however, exceed i m. grm. per 100
grams of the acid ; no trace of arsenic, iron or chlorine should be present.
According to the Italian Pharmacopoeia, 10 c.c. should leave no trace of residue
on evaporation.

HYDROFLUORIC  ACID

HF = 20

Aqueous solutions containing 60-65% by weight of HF (D = 1-23-1-263)
or 50% or 40% (D = 1-189 ~ I'I5I) are s°ld- ^ *s a colourless liquid,
fuming in the air, of irritating caustic odour. It is kept in vessels of gutta-
percha, hardened rubber or, better, of paraffin. It may be contaminated
with sulphuric, hydrochloric, nitric and hydrofluosilicic acids, arsenic, heavy
and earthy metals and organic matter (derived especially from the vessels).en the two weights the acetic acid is calculated.                        -jl ,